The Mannesmann tube-making process is widely employed as a process for manufacturing seamless tubes. In the Mannesmann tube-making process a round billet that is heated to approximately 1200° C. is pierced and rolled using a piercer. The piercer includes a pair of inclined rolls and a plug. The plug is disposed on a path line between the pair of inclined rolls. In the piercer, a round billet is pressed to the plug while being rotated in a circumferential direction using the inclined rolls. As a result, the round billet is pierced and rolled so as to form a hollow shell. Furthermore, as necessary, the hollow shell is drawn and rolled in an elongator so as to increase the diameter and to decrease the thickness. The elongator has the same configuration as the piercer, and includes a pair of inclined rolls and a plug.
As described above, die plug is used in the piercing-rolling of high-temperature round billets, the drawing-rolling of hollow shells, and the like. Therefore, heat and high contact pressure are delivered to the surface of the plug from round billets or hollow shells. As a result, wear or seizure occurs on the surface of the plug, or melting loss occurs on the plug. The service life of the plug is shortened due to the wear and the seizure which occur on the surface of the plug and due to the melting loss which occurs on the plug.
Generally, on the surface of a plug main body (base metal of the plug), an oxidized scale coating is formed. The oxidized scale coating shields heat from round billets so as to suppress the occurrence of the seizure or the melting loss. For example, Patent Documents 1 to 5 disclose plugs including an oxidized scale coating.
However, the oxidized scale coating has low wear resistance. Therefore the oxidized scale coating is worn each time a round billet is pierced and rolled. When the thickness of the oxidized scale coating decreases due to wearing, the temperature of the plug main body increases. As a result, the seizure or the melting loss occurs on the plug. Particularly in a case in which the plug is used in the piercing-rolling of and billet made of a high alloy containing a large content of chromium and nickel, the oxidized scale coating is significantly worn.
In a case in which the oxidized scale coating is worn, it is possible to form an oxidized scale coating again on the plug main body by carrying out a heat treatment. However, since the heat treatment takes a long time in a range of several hours to several tens of hours, the production efficiency decreases.
Patent Documents 6 to 9 disclose plugs including a coating other than the oxidized scale coating.
A plug disclosed in Patent Document 6 includes an arc-sprayed coating made of an iron oxide. The arc-sprayed coating has better wear resistance than the oxidized scale coating.
Plugs disclosed in Patent Documents 7 to 9 include a build-up layer on the surface. The build-up layer has excellent high-temperature strength and excellent wear resistance.